1. Field of the Invention
The present invention relates to a lens unit and an image sensing apparatus incorporated with the lens unit, and more particularly to a lens unit for use in a single-eye camera incapable of exchanging a lens unit.
2. Description of the Related Art
The single-eye camera is generally provided with a high-magnification zoom lens unit capable of zooming in a wide range from a wide-angle end to a telephoto end, in view of a point that the single-eye camera is incapable of exchanging a lens unit. In order to realize a high zoom magnification, and reduce the lens barrel length in a retracted state for miniaturization, the lens barrel has plural step portions, and the moving amount of a lens group i.e. the protruding amount of a lens barrel is increased. Further, there is a demand for increasing the speed of protruding the lens barrel. As a result, the pressure angle of a cam or a helicoid is increased. For instance, a zoom magnification such as ten times or thirty times is realized by about ⅓ to ¼ turn of a cam cylinder. On the other hand, the lens diameter and the weight of a first lens group (including a forwardmost lens element) are increased to reduce the focal length on the short focus side for realizing a high zoom magnification.
A multi-stage gear is provided to drive a load whose magnitude has been increased for the above reasons. As a result, backlash resulting from a zoom operation is increased. Normally, in the case where a lens unit is directed upwardly or downwardly, a lens driving mechanism is freely moved within a backlash range of a zoom operation. When the driving mechanism is freely moved, a smooth driving operation for a zoom operation is obstructed, which may lower the commercial value of the lens unit. In a fixed mode where the focal position is fixed, the focal position may be displaced, or a measurement error may be included in distance information for auto-focus control, with the result that a defocus may occur. The drawback becomes conspicuous by an increase in the lens weight, the pressure angle, or a backlash amount resulting from a zoom operation.
To suppress the free movement, there is proposed a method, wherein a guide pin of a cam cylinder is urged by a compression spring, and the guide pin is pressed against the cam cylinder. The technique, however, may increase a load necessary for driving the cam cylinder, and generate dusts resulting from abrasion of the cam cylinder.
As a method for suppressing backlash resulting from a zoom operation, there is proposed a method, wherein a suede cloth is attached e.g. between a cam cylinder and a fixed cylinder to prevent free movement of the cam cylinder by abrasion against the suede cloth. The technique, however, has a drawback that the suede cloth is flattened, and is likely to wear over time.
There are methods as disclosed in JP 2005-283677A (D1) and U.S. Pat. No. 7,230,773 (D2), as other methods for suppressing backlash resulting from a zoom operation. In the methods disclosed in D1 and D2, backlash is eliminated by mounting a coil spring between lens groups. In the methods, however, there is a likelihood that a cam cylinder may be freely moved by a spring force of the coil spring, depending on an arrangement of an optical system. Thus, the methods may be impractical in the case where a change in position between lens groups is relatively large, and obstruct miniaturization.
On the other hand, JP 2003-279827A (D3) discloses eliminating backlash by providing a coil spring of a diameter substantially equal to the diameter of a cam cylinder at an end surface of the cam cylinder, and urging the cam cylinder in the optical axis direction by a spring force of the coil spring.
In the conventional art disclosed in D3, however, it is necessary to provide a coil spring of a diameter substantially equal to the diameter of a cam cylinder. Accordingly, the length of the cam cylinder is unavoidably increased, which not only obstructs miniaturization, but also increases the difficulty in assembling the lens unit. Thus, the method is not desirable in the aspect of productivity.